Whistles are widely used as an easy-to-use communication method using a sound in various fields, such as referees in various sport competitions, guarding, guiding and signaling in a place where many people gather, or training pets. Whistles are required to immediately deliver a sound to a person or an animal around thereby to alert the person or animal, according to each environment where the whistles are used. Therefore, it is required for a whistle to be able to be immediately and easily blown when needed. Meanwhile, for a listener, the sound is required to have an easy-to-hear sound quality and can be immediately recognized.
Pealess whistles can generate a sound with a higher frequency and a faster attack, compared with pea whistles including an oscillator within a resonance chamber. Therefore, pealess whistles are mainly used in a so-called professional field, such as traffic control and sport referees requiring an immediate communication and alerting. In such pealess whistles, whistles with three resonance chambers having different lengths become mainstream in order to exploit the aforementioned features. That is because similar but different three sound waves are interfered with one another thereby to generate a beat sound. Two sound waves can generate a beat sound, but three sound waves can generate a richer and clearer beat sound. The beat sound has an important role of making the sound of a whistle stand out from ambient noise. In order to generate an effective beat sound, it is important that respective sound waves are clearly generated, the frequencies of the respective sound waves are close and also the amplitudes of the respective sound waves are close. Since the amplitude of a generated sound wave depends on a width (cross-sectional area) of a resonance chamber, it is desired that the widths (cross-sectional areas) of the respective resonance chambers are identical.
As a pealess whistle with such three resonance chambers, the following whistles are disclosed in U.S. Pat. No. 5,086,726, Unexamined Japanese Patent Application KOKAI Publication No. H1-65598, and U.S. Design Pat. No. 409,939.